devenv: override temp dirs after nix develop

`nix develop` overrides all of the temp directories, which causes
problems for module evaluation after the shell, for example `devenv up`.

The temporary `nix-shell-<hash>` directory it creates is also unsafe to
use as it might be garbage collected at a later point.

The previous solution was to pick up the `DEVENV_RUNTIME` var, but that
is not safe for nested devenvs and does not solve the the tmpdir issue.

src/modules/top-level.nix

@@ -186,16 +186,11 @@ in
         # - free to create as an unprivileged user across OSes
         default =
           let
-            runtimeEnv = builtins.getEnv "DEVENV_RUNTIME";
-
             hashedRoot = builtins.hashString "sha256" config.devenv.state;
-
             # same length as git's abbreviated commit hashes
             shortHash = builtins.substring 0 7 hashedRoot;
           in
-          if runtimeEnv != ""
-          then runtimeEnv
-          else "${config.devenv.tmpdir}/devenv-${shortHash}";
+          "${config.devenv.tmpdir}/devenv-${shortHash}";
       };
 
       tmpdir = lib.mkOption {
@@ -268,6 +263,16 @@ in
     enterShell = ''
       export PS1="\[\e[0;34m\](devenv)\[\e[0m\] ''${PS1-}"
 
+      # override temp directories after "nix develop"
+      for var in TMP TMPDIR TEMP TEMPDIR; do
+        if [ -n "''${!var-}" ]; then
+          export "$var"=${config.devenv.tmpdir}
+        fi
+      done
+      if [ -n "''${NIX_BUILD_TOP-}" ]; then
+        unset NIX_BUILD_TOP
+      fi
+
       # set path to locales on non-NixOS Linux hosts
       ${lib.optionalString (pkgs.stdenv.isLinux && (pkgs.glibcLocalesUtf8 != null)) ''
         if [ -z "''${LOCALE_ARCHIVE-}" ]; then